An usual electrophotographic image forming apparatuses are susceptible to an usage state of each process portion, surrounding environmental situations and the like, due to the principle of operation that image formation is performed using static electricity and therefore, they have a problem that a toner density of a formed image is also liable to change and as a result, image quality is liable to deteriorate. Against these circumstances, in Japanese Laid-Open Patent Publication No. 2003-15369, an electrophotographic image forming apparatus is developed, in which an adjustment step of adjusting control conditions (charging voltage, exposure amount, developing bias, transfer bias and the like) in each portion of an image forming portion (process control) is provided and the adjustment step is carried out in accordance with predetermined conditions, and thereby image quality is adjusted so as to be able to obtain excellent image quality at all times.
In the above-mentioned image quality adjustment, a toner patch for a test is formed on an image carrier such as a photoreceptor, an intermediate transfer body or a transfer body at a predetermined timing other than forming images, and the above control conditions are adjusted based on the value of an image (toner) density of the toner patch measured by a density sensor.
In a process control mode, a toner patch serving as a density reference is formed on an image carrier (including an intermediate transfer body such as a photoreceptor, an intermediate transfer drum or an intermediate transfer belt, or a transfer body such as a transfer drum or a transfer belt), a density of the toner patch (toner adhesion amount) is detected by an image sensor that is an optical detection means, and based on the detected density, feedback is applied to parameters (image forming conditions), such as charging voltage, exposure intensity, developing bias potential, transfer voltage and amount of toner supply (in the case of two component development) which affect the image density.
On the other hand, Japanese Laid-Open Patent Publication No. 10-272806 discloses a technology that in a color image forming apparatus, a color sensor for detecting a sheet color is provided near a sheet supply port, and at the time of performing color printing, color tones of a sheet to be printed are detected by the color sensor, so that optimization of more precise color adjustment is applied depending on the color of the sheet to be printed.
However, in a color image forming apparatus provided with a function of detecting a toner patch image by a toner density sensor to perform image quality adjustment, a color sensor for detecting the sheet color needs to be further provided on a sheet transport path in order to add a function of applying optimization of color adjustment depending on a sheet color, but this inhibits miniaturization of the apparatus.